[600MRG] Rx noise 630m

Sun Mar 19 13:30:43 EDT 2023

Hi Dave,

I see what you are doing now. I thought you were looking at site or band 
noise changes.

You are trying to determine if your loop antenna is setting the noise 
floor of your system or if the amplifier and receiver system are 
limiting the noise floor.

The ideal test of course would not be to dead short the loop amplifier 
input, but to substitute a dummy load with an equivalent impedance to 
the loop. If the noise drops several dB at the quietest time of day in 
the most quiet season at the narrowest receiver bandwidth you know the 
loop external signal pickup (absent any common  mode) is setting the 
noise floor.

The problem with a short is mostly in the amplifier behavior. We don't 
know how the amplifier and any matching system reacts reacts to a dead 
short, so that may or may not skew results enough to mislead us. 
Removing the antenna and substituting a more similar inductive load 
might be better.

I was going to put a loop or a voltage probe out back in the woods. I 
mostly receive on my Beverages, and I don't use the receiver filter 
bandwidth WSJT is based on, so the already nearly useless SN reports in 
WSJT are skewed a lot more than they could be.

My inverted L antenna noise level measurements like any antenna system 
noise level measurements are influenced by the inverted L efficiency and 
pattern. Since this is a full size antenna the only real losses are 
ground related losses. Matching losses are zero.

I know I'm close to limiting by ionosphere propagated distant noise at 
night on the L because because on the quietest winter nights by noise 
floor increased about 3dB rather consistently. I'd like to see that be 
10dB or more.

If I consistently saw no change day or night then I would know local 
noise is dominating my system.

At 0600Z last night my L antenna's 3.1 kHz bandwidth noise level was 
-68dBm average with peaks over -20 dBm from distant lighting storms. The 
530 kHz AM BC channel was -23.5 dBm average.

My daytime local solar noon noise floor was  -72dBm and the 530 kHz 
channel (all sky wave propagated) signal level is -62 dBm.

I have to understand my ambient and propagated noise so I know how much 
negative antenna gain I can tolerate when building a directional receive 
antenna. I am limited to 1000 feet or so in every direction except 
NE/SW.  I have to use a "compact" array since I have less than 1/2 wave 
of space going NW and SE, and that means significant negative antenna 
element gain. This is why I am trying to understand the band noise floor 
here.

73 Tom

On 3/18/2023 5:08 PM, Dave Riley via 600MRG wrote:
> TNX, Tom for another look through your prism..
>
> I just set the Cushman 24 up on 475kc.  With a shorted input it reads 
> -111 db ( not sure of cal )
>
> Then I plug in the loop and it goes up to -5 on the Cushman or +6 db 
> over -111 noise.
>
> I'd guess that the -111 is the noise floor on that meter.  It was on 
> 3.1 kc
>
> I tuned off to 460 and then 490 and the noise indicated -110 db on the 
> edges.
>
> Tuned to 476 kc. and it was -5  ' fairly good loop Q '
>
> Best I can do is 'relative'...  Think I should be using a 50 ohm load 
> instead of a short..
>
> TNX summore de DaveR W1FRV
>
>
>
>
> On 3/18/2023 4:09 PM, Tom W8JI wrote:
>> Hi Dave,
>>
>> Relative performance or noise performance is very difficult to 
>> measure in a meaningful way.
>>
>> 8dB is meaningless. 8 dB is just a ratio, what is the 8dB ratio 
>> against and under what condition?
>>
>> The 13dB difference between a short and an antenna doesn't mean very 
>> much of anything either. It only means the antenna is picking up 13dB 
>> more noise than a short. That doesn't mean anything about ambient 
>> noise or antenna receiving performance. It only means is the antenna 
>> system has 13dB more background noise than a short in some test. It 
>> doesn't tell us anything about how well something is working, how 
>> poorly it is working, or what the ambient noise level is unless there 
>> is some good reference.
>>
>> Now if we have some reference signal it might mean something. My full 
>> size inverted L antenna in January was about -73 dBm in 3.1kHz 
>> bandwidth daytime at this time of day, and -70 dBm average on a quiet 
>> night just before sunrise.
>>
>> I just measured it now and it is -72 dBm in the same 3.1 kHz 
>> bandwidth. Some digital signal on right now is -86 dBm, 14 dB below 
>> the 3.1kHz noise floor.
>>
>> My guess is that is N4WLO, my level meter can't decode digital signals.
>>
>> Even this does not mean much.
>>
>> 73 Tom
>>
>>
>>
>>
>> On 3/18/2023 3:11 PM, Dave Riley via 600MRG wrote:
>>
>>> Saying good bye to seasonal DX..
>>>
>>> Think the bulk of QRN has been nulled out here but still, the 
>>> difference between antenna and a short is about 13db
>>>
>>> Not much difference in noise level with loop spun around either..
>>>
>>> Maybe there is 8 db ambient noise average, some say that is low..
>>>
>>> What you say??
>>>
>>> TNX Dave @ W1FRV
>>>
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